The protest gathering was held outside the investigation and prosecution bureau in the town of Buraidah on Friday.
Fifteen women are reported to be among those detained. They demanded the release of female political activists recently arrested in the province.
According to state news agency, SPA, the sit-in protest lasted for more than 12 hours.
Saudi activists say there are more than 30,000 political prisoners, mostly prisoners of conscience, in jails across the Kingdom.
According to the activists, most of the detained political thinkers are being held by the government without trial or legitimate charges and have been arrested for merely looking suspicious.
Some of the detainees are reported to be held without trial for more than 16 years. Attempting to incite the public against the regime and the allegiance to foreign entities are usually the ready-made charges against political dissidents.
In Saudi Arabia, protests and political gatherings of any kind are prohibited.
In September 2012, then Interior Minister Prince Ahmed bin Abdulaziz said no further protests about detainees would be tolerated and that the Kingdom would deal "firmly" with protests.
Amnesty International has criticized Riyadh over the warning and urged the authorities to "withdraw their threat."